A screw-type extruder of this kind has become known from DE 40 10 540 C1.
Screw-type extruders serve for plastifying and homogenization of the mass to be extruded, a process involving strong heating and depending on the nature of the mass. In the case of cross-linkable/vulcanizable masses this heating limits the throughput rate of the extruder, since cross-linking and/or vulcanization are not allowed to occur in the extruder. Among the technical means achieving the required plastification with a minimum heating of the mass, in practice two principles have been found to be particularly effective, namely the so called transfer mix according to DE 27 31 301 A1/DE 27 31 438 A1 and the pin extruder with radial pins according to DE 22 35 784 B2 and others. Radial pins with an active conveying or mixing action, e.g. according to DE 36 13 612 A1 are substantially more sensitive from the mechanical point of view than simple radial pins and have therefore not been introduced in practice.
However besides the natural wear, the biggest disadvantage of pins radially projecting into the cylinder is that the pins even when new, but especially after a certain wear, are subject to breakage. Such pin breakages are costly, since they do not only damage the machine but also create product problems because they cannot be covered by the product liability insurance of the manufacturers and result in rejects or high control costs.
The extruder according to DE 40 39 942 C1 which is supposed to combine the advantages of the transfer mix and of the radial-pin extruder has also the same problems and in addition is also very expensive.
The tangential-pin extruder according to DE 40 10 540 C1 avoids the breaking danger due to the different, namely, tangential arrangement of the pins, lowers the manufacturing costs compared with the transfer mix and allows for an intensive tempering of the pins, which makes possible a higher rotational speed of the screw and thereby higher throughput rates of the extruder. These pins, however, also do not have an active conveying effect. A disadvantage similar to the radial-pin extruders is that self-cleaning does not occur without problems. The insertion and removal of the conveyor screw is possible only when the pins have been pulled out from the inner cylinder space.